The pigment suspension of the present invention is used for producing a film coating of such items as pharmaceutical tablets, confectionary pieces, and the like. The pigment suspension is typically stirred into a larger volume of polymer solution. The resulting film-forming suspension is used in the coating process. The film coating, in the form of a very thin film, must be uniform and consistent from one batch of tablets to the next.
The technique of film coating is generally known in the prior art. U.S. Pat. No. 2,954,323 to Endicott et al. discloses the increased efficiency and superior coating properties obtained with film coating in general as compared to other processes of coating.
The present invention relates to a pigment suspension which comprises, as pigment, titanium dioxide. Such pigment suspensions for use in film coating are preferably sold having a concentration of titanium dioxide as high as possible. However, as the concentration of titanium dioxide increases, the suspension tends to become more viscous and may reach a point where it becomes difficult to pour from its container. Upon aging, a thick suspension of titanium dioxide may harden to the extent of becoming unusable.
In developing a high concentration pigment suspension, it is desirable to obtain a product in which the titanium dioxide particles form a stable suspension and will not settle for a prolonged period of time. The need is for a pigment suspension which will readily pour from its container and will maintain its uniform properties, during both transportation and storage, until ready for application in film coating.
U.S. Pat. No. 3,981,984 to Signorino discloses a pigment suspension which claims to achieve a high concentration of titanium dioxides in a non-aqueous solvent. This pigment suspension consists of titanium dioxide particles, a protective colloid such as hydroxypropyl cellulose, and a non-aqueous solvent such as ethanol. Signorino teaches that as the titanium dioxide particles are added to the solvent, the mixture becomes too viscous, and the further addition of the protective colloid serves to suspend the particles and reduce the viscosity.
In view of the increasingly strict requirements of governmental regulating agencies in regard to the use of organic solvents, it has become desirable to obtain aqueous pigment suspensions. However, a high content of titanium dioxide in water is not normally possible for use in film-coating. Although titanium dioxide suspensions in an aqueous sugar syrup are known, such suspensions are not generally suitable for use in a film-forming polymer solution.
The present invention involved a search for a combination of ingredients which would permit a high content of titanium dioxide particles in an aqueous suspension useful in film coating. Due to the fact that the composition may comprise merely water and a very small amount of xanthan gum, not requiring the presence of organic solvents, the composition is very simple, safe and inexpensive to make.